The invention relates to the recovery of oil from subterranean formations by surfactant flooding operations. More particularly, the invention concerns the use of ethoxylated phenol and the ethoxysulfated phenol additives to polymer slugs which permit the use of a high salinity polymer drive following a surfactant slug.
Investigations of ways to increase oil recovery by improving the displacement ability of a water flood have produced useful surfactants which reduce the interfacial tension between the oil and water in the reservoir. With lower interfacial tension, oil that is trapped in the pore structure can disperse into the water as smaller and more easily deformable droplets. Many types of surfactants have been investigated and the choice of which surfactant to employ in a water flood operation is dependent upon the conditions in the reservoir, as well as the cost and availability of the surfactants.
Most water flood operations have employed a petroleum sulfonate as a sole surfactant, or at least a major component of a mixture of surfactants. Synthetic alkylaryl sulfonates and alkyl sulfonates and sulfates have been proposed as oil recovery surfactants. Sulfonates are preferred because they have a better high temperature stability than the sulfates. These surfactants are all classified as organic sulfonates and are usually metal salts of alkylbenzene sulfonate containing 12 to 30 carbon atoms, but may also be aliphatic sulfonates or alkylated naphthalene sulfonates. These surfactants have an equivalent weight that ranges from 320 to 700 g/mole.
To combat layering and precipitation problems in high salinity applications, a material with both water-soluble and oil soluble characteristics is usually added to organic sulfonate surfactant mixtures. When used in surfactant flooding, these materials are generally referred to as "solubilizers" or "cosurfactants" and often constitute the most expensive component in a surfactant mixture. Conventional solubilizers are sulfate or sulfonate salts of polyethoxylated alcohols or alkyl phenols. The amount of solubilizer required depends on the amount of and types of organic sulfonate surfactants employed in the water flood operation. A minimum amount of solubilizer is required to prevent the surfactants from precipitating from the flood water The choice of the solubilizer employed is dependent on the choice of surfactants to be used and the salinity of the flood water. Surfactant quantity is a function of the reservoir's size and other characteristics. The concentration of surfactant components in the system is usually 1% to 6% expressed on an active surfactant basis. The surfactant slug is usually made up in high salinity brine and the polymer slug in fresh water.
Generally, a surfactant slug is followed by a polymer drive fluid or a polymer slug. Unfortunately, surfactant slug formulations invariably perform more efficiently when followed by fresh water polymer slugs. Because of the higher cost and limited availability of fresh water, the industry has devoted considerable effort in attempts to discover surfactant and polymer systems which will perform well with high salinity surfactant and polymer slugs.
U.S. Pat. No. 3,437,140 disclosed a postflush to a surfactant slug which contained a relatively high salinity. This postflush slug contained some of the surfactant from the surfactant slug to build a second bank of surfactant to displace additional oil. U.S. Pat. No. 3,637,017 taught a method of adding alcohols such as isopropyl alcohol to slugs following surfactant slug to reduce surfactant depletion. But the system was not effective when salinity reached 1.5% sodium chloride. A "chasing surfactant" between the micellar solution and the aqueous drive was disclosed in U.S. Pat. No. 3,990,515 as a brine soluble anionic dispersing agent which was more water-soluble than the original surfactant. The chemicals mentioned were alpha olefin sulfonates, ethoxysulfates, and dialkyl sulfoccinates.
An aqueous postflush slug containing a nonionic solubilizer to reduce surfactant loss and improve oil recovery was disclosed in U.S. Pat. No. 4,276,933. The solubilizers used were ethoxylated and propoxylated alkyls having 9 to 16 carbon atoms and 1 to 20 ethylene oxide or propylene oxide groups. A similar approach was taught in SPE Paper No. 14292 by Minssieux, L., "Surfactant Flooding With Hard Water: A Case Study Solved by HLB Gradient," presented at the SPE 60th Annual Technical Conference in Las Vegas, Sept. 22-25, 1985. This solution to a high salinity drive problem injected a desorbing agent with the polymer slug having a higher hydrophile-lipophile balance (HLB) than the nonionic cosurfactant in the surfactant slug. The nonionic additives were disclosed as ethoxylated alcohols, specifically C.sub.16 -C.sub.18 (EO).sub.6 in the surfactant slug and C.sub.16 -C.sub.18 (EO).sub.20 in the high salinity polymer drive.